A typical automotive type solid electrolyte exhaust gas oxygen sensor is disclosed in U.S. Pat. No. 3,844,920 Burgett et al. It has a zirconia sensing element shaped as a tapered thimble. One end of the tapered thimble is open and has a thick circumferential flange. The other end is closed and forms the most active part of the element. The interior and exterior of the thimble each has a discrete porous electrode coating of platinum or the like.
The interior, i.e. reference or inner, electrode is exposed to a known source of oxygen, such as air or a mixed metal oxide, for establishing a reference electrode potential. This electrode is generally formed by thick film techniques, such as painting a coating of platinum ink onto the zirconia thimble, drying the coating, and then firing the coated thimble at an elevated temperature. An improved technique for applying the reference electrode, is disclosed in U.S. Pat. No. 4,264,647 Trevorrow.
The exterior, i.e. exhaust or outer, electrode is usually formed by thin film techniques, such as evaporation or sputtering. Improved sputtering techniques for applying the outer electrode are disclosed in U.S. Pat. Nos. 4,244,798 Gold et al; 4,253,931 Gold et al; and 4,303,490 Gold et al. Both Gold et al patents describe sputtering from a planar target that is part of a commercially available DC magnetron cathode assembly. They do so at a pressure of about 10-20 millitorr, preferably in an atmosphere of nitrogen and/or oxygen along with argon, a target-thimble spacing of at least about 3.0 cm, and at a high sputtering rate of 13-20 watts per cm.sup.2 of target area, to provide fast responding sensors. U.S. Pat. No. 4,253,934 Berg et al describes increasing the yield of fast responding sensors by treating the electroded thimbles in substantially oxygen-free nitrogen at an elevated temperature. However, there was no practical way to discern a fast responding thimble from a slow responding thimble before assembly into the finished sensor. Accordingly, Berg et al proposed nitrogen aging all the thimbles before assembly into sensors. The increase in yield of fast responding sensors was sufficient to offset the cost of such a treatment.
In my U.S. Pat. No. 4,400,255 Kisner, I describe improving the foregoing techniques of sputtering from a planar target even further. I describe supporting the thimbles during sputtering in such a way that the inner and outer electrodes can be maintained at relatively different electrical potentials. In addition, I disclose providing a low resistance electrical path between the inner electrode and the sputtering anode.
I have now invented a new sputtering cathode assembly that is distinctly different from the planar target-cathode assembly previously referred to. It is described in my U.S. patent application Ser. No. 625,847 that is entitled "Magnetron Sputtering Cathode Assembly and Magnet Assembly Therefor" and filed concurrently herewith. My new sputtering cathode assembly provides faster platinum deposition and stronger heating than the planar DC magnetron cathode assembly previously used to deposit the exhaust electrode on the exhaust oxygen sensor. Moreover, I believe my new cathode assembly provides more uniform heating than the commercially available planar DC magnetron cathode assembly previously used. In brief, my new DC magnetron cathode assembly provides a considerably stronger magnetic field across the target face and a much stronger heating effect than attained with the planar DC magnetron cathode assembly previously used. I believe that its effects are more uniform too.
My new DC magnetron cathode assembly must be used in a new way if one wants high yields of fast responding exhaust electrodes when using it. This specification describes and claims the new process of using my new cathode assembly.